Can you vacuform over (poly)urethane foam?

[Ein]

Pretty much as it says in the title. I have something I can use a negative mold to fill with urethane foam - I'm thinking the 8lb stuff - and I want to know if I can try a PETG pull overtop of that foam without destroying it. Anyone have any idea? I have no idea where specifically to ask but if anyone knows, I expect it's the folks on this forum.

 

[cavx]

If you have a female tool (negative mold), why not just vacuum form into that? Obviously, it depends on what that female tool/mold is made from.

Female tooling works much better and almost completely removes all of the frustrating fail points of traditional male vacuum forming you read about on this forum.
You get a much tighter pulls and you can use thicker plastic for a more hard use-able part. Most positive parts result in paper thin wall thicknesses. When they increase the plastic thickness, they lose detail.

Female tools pull the plastic into the tools, so you get all the detail regardless. You-Tube the technique. It is very cool and they make spar baths this way.

 

[laellee]

I would take another piece of the same foam and do a test-pull on that. If it sticks (because of the heat) and bonds on, then you'll at least be able to figure out your options from there without ruining your good buck.

As an aside, if it does fuse to the foam, there is another option you can pursue... If you try and vac-form styrene over a material like monster clay, the styrene will fuse right onto the clay buck due to the heat, effectively wrecking the buck if you try to separate them again. What you can do, however, is now pull a SECOND piece of styrene over the first fused layer; it will release cleanly from the first layer, hopefully with a minimum amount of detail lost through the extra styrene on the buck. Just a thought, anyways.

 

[cavx]

@<i><a href="http://www.therpf.com/member.php?u=35259" target="_blank">Ein</a></i>, I think you need to watch this video.

- - - Updated - - -

And this one too.

Notice the fine details that can be had from this technique that you won't get from male vacuum forming.

 

[Ein]

I would take another piece of the same foam and do a test-pull on that. If it sticks (because of the heat) and bonds on, then you'll at least be able to figure out your options from there without ruining your good buck.

This is always the smart option, but at the moment I don't actually have the foam in question. I'm trying to determine what the best material to make the buck out of is that gives me the easiest carvability without also giving me a gigantic headache.
cavx That first video you linked is really interesting - definitely a good proof for why female molds work so well. In that first video, does he just have a multitude of tiny drilled holes in the actual female mold itself? I don't see where the vacuum is being made.

For context's sake, I'm not actually after that much detail on this particular job:

It's a more complicated and organic shape than I'd like to try and get out of a wood or MDF buck, as I have a feeling I'd be losing my ******* mind sitting there with a dremel trying to match the symmetries across the midsection. Seemed to me that if I could get some kind of foam, it'd be the easiest material to carve to the right shape, but foam will itself get totally destroyed. The idea of pulling a second sheet of plastic over the first one is really interesting - I guess if you apply a wax paste or something to the bottom sheet, the top one would still come off?

 

[cavx]

@cavx That first video you linked is really interesting - definitely a good proof for why female molds work so well. In that first video, does he just have a multitude of tiny drilled holes in the actual female mold itself? I don't see where the vacuum is being made.

It depends on the amount of suction and what the tool is made from. Generally 1 single hole is all that is needed.
In the case of a vacuum pump and MDF, no holes are required. Once suction is greater than 6 inches of mercury , the air is pulled right through the MDF. Therefore the outside case needs to sealed with Fibre glass or you only ever pull 6"Hg.

 

[laellee]

...It's a more complicated and organic shape than I'd like to try and get out of a wood or MDF buck, as I have a feeling I'd be losing my ******* mind sitting there with a dremel trying to match the symmetries across the midsection. Seemed to me that if I could get some kind of foam, it'd be the easiest material to carve to the right shape, but foam will itself get totally destroyed. The idea of pulling a second sheet of plastic over the first one is really interesting - I guess if you apply a wax paste or something to the bottom sheet, the top one would still come off?

Nope, nothing so complicated. It at first seems counterintuitive (I was taught this from someone else), but no release is needed at all. Your secondary pull of styrene will pop right off the original pull once it's cooled. So no waxes, no release, nothing.

 

[paddyfritz]

We make dash boards at work. We vac form the skins, then fill them with PU foam after in a second mold if that helps any.

 

[cavx]

We make dash boards at work. We vac form the skins, then fill them with PU foam after in a second mold if that helps any.

Do you make them using a male or female tool?

 

[paddyfritz]

Do you make them using a male or female tool?

We make the skins on a male mold. The foam is added in a female mold. The injection mold has a abs backer placed on the top, and the skin is placed on the bottom. They are held in place by vacuum and clamps. Then the two mold halves close and the liquid foam mix is injected under pressure. It cooks for 45 seconds or so, then the mold opens.
It takes a while for them to cool off and fully cure.

The skin is the visible part of the padded dash. The abs backer is what snaps into the steel dash frame.